The VPN concept allows private traffic traditionally served by leased private lines to share the existing trunk capacity of the PSTN (Public Switched Telecommunication Network). This allows for a more efficient utilization of network resources and provides a number of benefits to both operators and users. Network costs can be more closely tied to actual usage. With private networks, the cost of intersite traffic is tied to the maximum capacity required, not overall usage. VPN also allows for increased flexibility in configuring the network.
QSIG (unified international corporate network signaling standard) is an ISDN common channel signaling system designed for use in corporate networks. It was developed in an open forum for all major European PBX manufacturers. The name QSIG refers to signaling across the ISDN “IQ” reference point which is the logical signaling point between two PINXs (Private Integrated Services Network Exchanges). For private ISDNs, only one protocol is necessary as the QSIG protocols have sufficient functionality to be used both within the network at transit nodes and outside at access nodes. Hence, QSIG can be used between all PINXs. In particular, QSIG defines a set of protocols for the interaction of PINXS, so that common sets of services and features, with similar appearances, are available to users across private networks, including multi-site networks.
A single QSIG port can be utilized to carry traffic to any or all other points in the VPN. The most common QSIG Access is a 2 Mbit/s interface which implements up to thirty 64 kbit/s B-channels and one 64 kbit/s virtually B-channel structure (30B+D structure). However, other access structures such as a 2B+D structure are possible, as well. In addition to supporting private traffic, the QSIG interfaces can provide a gateway functionality to carry traffic to and from a PSTN, resulting in increased efficiency in the provision of access ports. The VPN is always customer-specific and a system must be configured to suit the particular customer's requirements. The provision of the VPN network may be based on a software product running on the respective network switches. The maximum number of VPN groups depends on the data standard for the switches involved. A VPN indicator in the signaling defines the context to be used.
The original standards for QSIG were developed by the ECMA (European Computer Manufacturers Association). Later, these standards were used as a basis for the ETSI (European Telecommunications Standards Institute). The main standards covering the Basic Call Protocol are Layer 2 ETS 300 402-1 based on ITU-T Q.920 and Layer 3 ETS 300 172 (for the PSS1 signaling system) based on ISO/IEC 11572, or EN 301 060 (for the DSS1+ signaling system).
In addition thereto, a standard which covers the Core Generic Functional Protocol for supplementary services for private telecommunication networks has been developed by ISO (ISO/IEC 11582) and given the ETSI identification ETS 300 239. This is the core protocol that the individual supplementary services use to avail of the transport services of the underlying layers.
A VPN product provides QSIG interfaces for interconnecting remote PINXs across the PSTN, either via a single exchange or via a number of exchanges in the network. Conceptionally, the entire exchange network is seen by the end PINXs as a simple transit network.
The transport of the QSIG signaling over the PSTN (e.g. ISDN network) is accomplished by “enveloping” QSIG messages within ISUP (ISDN User Part) messages. This method enables common ISUP signaling links and associated bearers to be provisioned for VPN and non-VPN traffic for greater efficiency.
To be able to offer a true on-net call and to provide for the transparent transfer of QSIG messages over the PSTN from the VPN entry point to the VPN exit point (when the VPN extends over multiple exchanges), a technique called Application Transport Mechanism (APM) is used. This means that a call is set up over the PSTN to provide for the connection, and all associated QSIG data is transmitted along the call with the APM mechanism. The receiving end extracts the data and sends an identical message to the destination PINX. Thus, PINXs see the network as a dedicated private network with QSIG signaling.
Since there may be parts of the PSTN that do not support the signaling, or the destination subscriber is not capable of supporting the protocol, a method has been suggested to drop the call to support only a basic call. When such a situation is detected, a so-called gateway function is performed and the necessary parts of the private network signaling are transformed to a PSTN signaling. If transparency is lost, i.e. the destination does not support QSIG, then the QSIG data is dropped from subsequent messages and a gateway function is performed in the network after this condition is detected. In particular, a gateway request indicating that no QSIG information transparency can be achieved is sent from a first network exchange or switch to a preceding second network exchange or switch, if the first network switch recognizes that the QSIG information flow cannot be continued due to a lack of network signaling capability. This Gateway Request procedure is defined in the ETSI specification EN 301 062-1 (Q.765-1), chapter 7.2.3.2.6, “Gateway PINX transformation request mechanism”. In addition thereto, the Gateway functionality of the QSIG-VPN is described in ETSI specification EN 301 069 and the ISO specifications ISO/IEC 11582 and ISO/IEC 11575.
However, according to the known method, the gateway request is forwarded to the originating network element, i.e. the respective Originating Local Exchange (OLE), such that the QSIG gateway functionality is always provided at the originating network element (OLE). Thus, QSIG-VPN network transmission load and service provision cannot be optimized by the public network operator.